Control of the characteristics of zinc oxide is realized by addition of additives and heat treatment. The Kroger-Vink notation of defects describes that electric conductivity is imparted by formation of defects according the following equations 1 and 2:ZnO→ZnZnx+Vo•+e′+(½)O2⇑  Equation 12ZnO→ZnZnx+Zni•+Vox+e′+(½)O2⇑  Equation 2
It has been thought that the defects induced as described above participate in luminescence of zinc oxide, and blue-green light emitting zinc oxide phosphors which have been developed and put into practical applications are regarded as including zinc oxide which contains defects synthesized by chemical reaction according to equation 1 or 2, i.e., a composition represented by the chemical formula Zn1+xO or ZnO1−x. For example, as disclosed in Patent Document 1, such a phosphor is produced by heat-treating an inorganic raw material containing zinc in a low oxygen-partial-pressure atmosphere.
Control of the characteristics of zinc oxide by addition of impurities is realized by adding any one of III group elements, such as aluminum, indium, and gallium, or an alkali metal element, such as sodium or lithium. The addition of additives is expressed by equations 3 and 4. Equation 3 indicates that electric conductivity is imparted by adding a III group element shown by X, and equation 4 indicates that electric conductivity is controlled by adding an alkali metal element shown by Z.ZnO+X2O3→ZnZnx+2XZn•+3O0x+e′+(½)O2⇑  Equation 3ZnO+Z2O+(½)O2+e′→ZnZnx+2ZZn′+3O0x  Equation 4
The effect of an additive on ZnO has been conventionally achieved by a general technique of introducing donors or acceptors, and basically, a green-light emitting zinc oxide-based composition has been realized by a composition represented by Zn1+xO or ZnO1−x, i.e., zinc oxide containing introduced donors.
However, in the present invention, the luminescence characteristics of zinc oxide are controlled by simultaneously adding a III group element which creates a donor level and an alkali metal element which creates an acceptor level in zinc oxide. Therefore, the present invention is different from the conventional technique.
A yellow-light emitting zinc oxide paint is disclosed in Patent Document 2. The invention described in Patent Document 2 discloses a paint composition which appears in yellow when illuminated with sunlight or white light representative of general illumination light, and a process for producing the paint. Namely, the invention of Patent Document 2 relates to a zinc oxide-based composition producing a yellow reflection spectrum. However, Patent Document 2 does not disclose luminescence from the yellow paint under illumination with particle rays or electromagnetic waves, and is thus different from the present invention in object and advantage. The yellowish luminescence cannot be achieved by the technique disclosed in Patent Document 2.
An example of a yellowish-light emitting phosphor is yttrium-aluminum garnet containing cerium. This phosphor is used as a scintillator material. For example, Nonpatent Document 1 shows the luminescence state of this phosphor. In recent years, the phosphor has been used for producing a white-light emitting semiconductor device by combination with a nitride semiconductor LED.
Apart from the green luminescence of Zn1+xO or ZnO1−x described above, there is also a process of producing visible luminescence using an additive. In this process, visible luminescence is activated by adding a lanthanide element. Patent Document 3 discloses a visible-light emitting composition. A zinc oxide-based composition of the invention of Patent Document 3 emits visible light using the emission line of a lanthanide element and is characterized by emitting light only at a specified wavelength. Therefore, this process is not a process for producing a zinc oxide-based composition giving a continuous emission spectrum with a broad luminescence band, which is representative of a so-called incandescent lamp. Therefore, the object of the present invention and the advantages achieved thereby cannot be achieved by the technique disclosed in Patent Document 3.    Patent Document 1 Japanese Unexamined Patent Application Publication No. 06-093259    Patent Document 2 Japanese Examined Patent Application Publication No. 06-070190    Patent Document 3 Japanese Unexamined Patent Application Publication No. 08-236275    Nonpatent Document 1 T. Tamura, T. Setomoto, and T. Taguchi, Journal of Luminescence, Vol. 87-89, P. 1180 (2000)